The present invention relates generally to electric power converter systems of the inverter type and more particularly to inverter systems having improved self-correcting control means.
It has been known for many years to provide variable power and frequency alternating current (a.c.) to a load through the use of an arrangement of conduction controlled devices. Originally, the devices employed in such arrangements were usually gas tube devices such as thyratrons but it is now much more common to use semi-conductor devices of the gate controlled type which are generically known as thyristors, the most common form of which is the silicon controlled rectifier. For convenience and purposes of this description, the term conduction controlled device will be used generally as a generic term while the term thyristor will be employed in the illustrated embodiment.
In one of the more common three phase versions of an inverter bridge, variable direct current (d.c.) power is applied to a six thyristor bridge having a pair of series connected thyristors associated with each leg of the three phase output. An inverter control supplies gate pulses to gate the thyristors on at the desired frequency for the inverter output. One very common control which is discussed in some detail in the detailed description which follows includes an oscillator which outputs pulses at a frequency six times that desired for the output of the thyristor (one pulse per thyristor per cycle). These pulses are applied to a recirculating shift register having six bit positions. The shift register is preloaded with a specific content and the contents of the register are shifted one bit position with the application of each pulse. The direction of shifting is determined by some form of control signal signifying whether the inverter is to run in the forward or reverse mode. In this prior art system, gating pulses are generated as a function of the instantaneous content of the register bit positions; e.g., if binary 1's are in bit positions 3 and 4, gate signals are initiated which are applied to the thyristors associated with those two bit positions to thus render those thyristors conductive.
Such systems are generally satisfactory but they do suffer from some major difficulties. First of all, the extra circuitry required to provide the parallel loading of the recirculating shift register adds expense. More importantly, however, if the bit pattern within the shift register is disturbed for any reason, such as spurious noise or transients within the system, then the bridge ceases to function properly and it is necessary to shut down the system and reload the shift register to reinitiate proper bridge operation. This problem will be discussed in greater detail in the detailed description which follows.